1. Field of the Invention
The present invention relates to a miniature motor exhibiting reduced sliding loss on the end face of an oil-impregnated bearing which receives a thrust load of a rotor generated due to rotation of the motor, as well as to a method for manufacturing the miniature motor.
2. Description of the Related Art
A miniature motor having a worm reduction-gear functions such that a drive torque output from the motor is transmitted to a worm via a motor shaft, from the worm to a worm wheel, which is a helical gear, and from the worm wheel to an external load via an output shaft of the worm wheel. Upon startup of clockwise or counterclockwise rotation of such a miniature motor having a worm reduction-gear connected to an external load, a thrust force of the worm joined to the motor shaft acts in such a direction as to withdraw the shaft from a motor casing. As a result, the motor rotates while a washer on the shaft is pressed against a bearing.
In such a case where a worm is employed in a torque transmission system for transmission of torque to an external load, rotation of a motor induces a thrust load, and in some cases the minimum startup voltage of the motor increases due to a sliding loss associated with a frictional resistance between a washer on a rotor and the end face of an oil-impregnated bearing subjected to the thrust load. When the motor involving such a sliding loss is to be started, a greater amount of power is required as compared with a case of a motor that does not involve such a sliding loss. Therefore, in the case where a miniature motor having a worm reduction-gear is used to drive, for example, an air conditioner damper for use in an automobile, the miniature motor has involved a problem of failure to start up when the battery voltage is low as a result of, for example, low temperature.